The measurement of a gas concentration with infrared technology is generally implemented by means of a non-dispersive method, i.e. by measuring absorption through an appropriately selected bandpass filter. Thus, the measured attenuated radiation transmitted through the gas layer will be an integrated value of transmissions existing over various wavelengths of this band. This type of equipment is described for example in publications U.S. Pat. No. 3,745,349 and HEWLETT PACKARD JOURNAL, September 1981, pp. 3-5: R. J. Solomon--"A Reliable, Accurate CO.sub.2 Analyzer for Medical Use". The former publication discloses the use of two sources of infrared radiation emitting through a sample gas but, for example, an optical gas filter is used for removing characteristic wavelengths of an absorbing gas to be measured from the radiation coming from the first radiation source. The purpose is to establish a reference source, the radiation intensity produced thereby and detected by a detector not being influenced by a gas mixture to be examined. On the other hand, radiation from the second radiation source decays or attenuates strongly as a result of a gas mixture to be examined, whereby the proportional signal changes can be used for determining the concentration of a gas component to be studied. The method is also relatively effective for correcting the influence of contamination collecting in a measuring chamber and also other adverse effects, if the radiation sources are located on exactly the same optical path with respect to the detector. The latter publication deals in even more detail with problems associated with the determination of carbon dioxide, taking into account e.g. other gases interfering with a gas component to be examined. The publication discloses a solution by using a rotating disc, containing a variety of filters for obtaining a reference value. The measurement with infrared technology of a single gas component studied in the above-cited publications is in a way sensitized to a desired gas component by using special, i.e. primarily gas-filled, optical filters and often also gas-filled, e.g. pressure-detection based detectors, the filling gas in both of these consisting of a gas component to be studied. Based on the above, the available prior art will now be studied in a bit more detail.
There are several means for sensitizing the infrared measuring to a given gas component or for increasing the selectivity of measurement. When the infrared radiation is detected by using pneumatic, acoustic or capacitive gas-filled detectors, the measurement in a way sensitizes in itself to the very gas component used for filling the detector, since absorption is only within the absorption bands and absorption lines of this gas component, so that the absorption of radiation and, the gaining of a signal take place. If the question is about a wide-band detector, such as a so-called thermopile, a solid state detector or a thermistor, the sensitization of measurement is generally done by means of a narrow-band optical filter, having its passband preferably selected within the range over which the presently examined gas absorbs. Another option is to set upstream of the detector a gas-filled chamber, which eliminates radiation from the wavelengths of the absorption lines of this particular component, or it is possible to use both filters together. For example, US-publication 5,036,198 discloses a device combining the use of two rotating filter discs, one filter disc being provided with gas-filled, measurement-sensitizing, optical filters and the other with narrow-band optical bandpass filters appropriately complementing the former. The use of a wide variety of chambers and bandpass filters enables the concurrent measurement of several gas components. A complicated measuring system tailored as described above has also distinctive drawbacks. The measurement is only accurate for those gases which have been accounted for in the measuring process. Any unknown gas component absorbing within the transmission bands of optical filters ruins the measuring accuracy and, as a matter of fact, in order to cause error, the unknown gas component need not even be optically active since the gases influence each other also through collision broadening.
The publication U.S. Pat. No. 5,055,688 discloses a way of using gas-filled pneumatic detectors for sensitizing a measurement to a given or several gas components. The apparatus set forth in the publication employs two gas-filled detectors, each being further provided with two separate gas chambers. In addition, between the detectors is fitted a bandpass filter for ultimately sensitizing the detectors to various gas components, such as CO and CO.sub.2. The bandpass filter is used for selecting such of the absorption bands of gases to be examined, which are spaced from each other and which provide an amplification stage connected to the detectors with a signal of the same magnitude, even though the mean gas concentrations subject to measuring were clearly different from each other. Generally, a problem encountered in measuring methods using gas-filled optical filters or detectors is that the detector, upstream of which lies an optical, strongly attenuating filter containing a gas component to be examined, has a signal which is very weak as compared to the signal of a so-called reference detector, which has no extra attenuation or absorption. This problem develops since the sensitization of a measurement to a predetermined gas requires a high selectivity or the major attenuation of radiation within the range of wavelengths over which the gas component to be measured absorbs. This difficulty is particularly pronounced in those measuring systems intended for measuring various isotopes of the same molecule, or in fact of a specified atom thereof. The US-publication 5,486,699 describes the measurement of various carbon isotopes by means of a solution, which involves quite a complicated chamber configuration and electronics for regulating in a special way the amplification and zero level of a signal produced by detectors.
The above-described prior art deals with such measuring systems, which employ gas-filled optical filters or detectors or optical bandpass filters in such a fashion that, in the vicinity of the absorption band of a gas to be analyzed, the influence of an absorbing, measurement-disturbing gas is eliminated or significantly reduced, or that the infrared measurement is accompanied by a reference measurement for recuding mechanical or e.g. contamination-induced influences, or that the measurement is performed on several different optically active gas components. The above-described prior art does not deal with changes occurring within a single absorption band of a gas molecule or, in practice, changes caused by collision broadening on the fine structure of an absorption band and an error resulting therefrom in the concentration analysis of a gas component.
The absorption spectrum of a gas in a molecular state consists normally of absorption bands produced by molecular vibrations and of a fine structure resulting from rotational transitions within the same. Thus, when measured with a sufficient resolution, the absorption spectrum of a gas consists of a large number of very narrow absorption lines. For example, carbon dioxide has a vibrational absorption band having a mean wavelength of 4260 nm. A more accurate analysis indicates that the range consists of more than 80 narrow absorption lines produced by rotational transitions. These lines have a half-value width and a relative height which depend on a plurality of factors, such as temperature, pressure, and even collisions by other molecules included in a gas mixture. Temperature and pressure can be generally accounted for simply by measuring the same and by correcting the measuring signal with this result. On the other hand, a change resulting from collisions of other gas components, having an indirect impact on the concentration analysis, must be accounted for in a special way. The publication APPLIED OPTICS, Vol. 25, No. 14, pp. 2434-2439, 1986: C. Cousin et al.--"Air broadened linewidths, intensities, and spectral line shapes for CO.sub.2 at 4.3 .mu.m in the region of the AMTS instrument" describes changes on the half-value width of carbon dioxide in a nitrogen and oxygen mixture. The carbon dioxide line (ordinal 67) of a normal-pressure gas mixture has a half-width value which in an oxygen mixture is 0.055 cm.sup.-1 (0.10 nm) and in a nitrogen mixture it is 0.060 cm.sup.-1 (0.11 nm) for a concentration of 5% CO.sub.2. The portion of a broadening caused by carbon dioxide to itself is only about 0.003 cm.sup.-1 in these figures.
The polar gases like laughing gas or nitrous oxide (N.sub.2 O) have still a lot more influence on the half-value width. Therefore, the measurement of a carbon dioxide amount in the respiratory gases of a patient is subjected to a laughing gas correction for example by measuring the N.sub.2 O concentration, as described in US-publication 4,423,739. The influence of oxygen as opposed to nitrogen is also often corrected, although the error is less serious. This type of correction method is not very good or even reliable but, in fact, the method requires that the concentrations of all gas components in a gas chamber and the influences of the gas components on collision broadening be known beforehand in order to perform the correction. The influences are not even unambiguous since the influence of a given gas in a gas mixture on the measurement may be different than the influence of the same gas alone. The measuring calculations necessitate highly extensive experimental information. The procedure described in the above-cited publication may provide a completely incorrect result, if a gas mixture to be analyzed is exposed to a previously unknown factor. As a result of such procedure, the measurement of all gases e.g. in a clinical patient monitoring situation will be generally highly expensive, if there is no other special reason for measuring these gas components.
The procedures described in the publication U.S. Pat. No. 3,745,349 and in other cited publications are not capable of solving the above problem without thoroughly understanding the phenomenon, nor have said publications even addressed the problem or sought a solution thereto. Hence, the above methods, intended for the sensitization of measurements to a given gas component, make use of a powerfully absorbing gas-filled band rejection filter, whereby the collision broadening and its influence cannot be measured. In addition, since the gas filter chamber is sealed and stationary as well as the fact that it contains a different gas composition and possibly a different pressure and temperature than those existing in the measuring chamber, the influence of collision broadening of absorption lines without a special dimensioning of the measuring chamber and filter chamber is difficult to account for. The accurate measuring results shall thus include errors, since the measuring systems as such do not tolerate a simultaneous measurement or correction of collision broadening during the course of measuring the concentration of a gas mixture.
The publication U.S. Pat. No. 4, 110,619 seeks to address the problem by attempting to completely eliminate the effect of collision broadening. The publication discloses both a so-called "dual-beam" arrangement, wherein a measuring chamber and a reference chamber and subsequent detectors are in parallel, and a so-called "single-beam" arrangement, wherein the detector is common for both signals and the signal of a measuring and reference chamber is separated by passing the radiation through the chambers after pulsing it e.g. with a chopper. The detector itself includes two separate, yet coupled-together, gas-filled chambers, wherein the absorbed radiation causes pressure changes that are measured e.g. with a sensitive microphone. The elimination of collision broadening is based on setting the gas sample of the detector chambers, the amount of gas, or the length of the chambers to be such that the composition of a gas mixture, i.e. the collision broadening effects, would not alter the actual measuring signal. As noted quite correctly by the author of the publication, the principle only applies to a sufficient degree in a so-called "single-beam" arrangement, but even in this the gas sample must be set such that, concurrently, the measuring sensitivity for a particular gas to be measured decreases, the stability of a null point suffers, and the sensitivity for measurement-disturbing other gases increases. In addition, since the so-called "single-beam" arrangement in fact includes three successive radiation-absorbing chambers (a measuring chamber and two detector chambers), which must be simultaneously brought to the required balance in terms of absorption, the compensation for influence of collision broadening is only possible for one given concentration of a gas to be measured. In the example disclosed in the publication, which describes a calibration procedure included in the method, this concentration is 10% CO.sub.2. Over other concentrations, the compensation for collision broadening is no longer satisfactory and, thus, the problem caused by collision broadening remains. The above phenomenon is due to the fact that the gas mixture in the measuring chamber also affects the radiation falling on the detector by changing its spectral shape at the absorption line. Since the adaptation of the amount of gas in the detector chambers is based on this very balance between the peaks and side slopes of an absorption line, which is thus also affected by a gas in the measuring chamber, there will be an error if deviation is made from a so-called calibration concentration. Hence, in a general sense, this type of compensation procedure cannot be a satisfactory solution for eliminating the error of a collision broadening. The use of pneumatic, bulky, and somewhat insensitive, and in this case also very specially designed detectors is as such unfit for modern times.
As simplified, the total absorption of gas is proportional to the number of gas molecules in a measuring volume. This is the case, especially if the sample chamber, and particularly its length, is small in the direction of infrared radiation. Thus, the inherent absorption of gas does not essentially alter the effect of radiation applied to each molecule. The radiation applied to a molecule may change as a result of the self-absorption of a gas to be measured in a gas mixture or other absorbing components within the same radiation band. The direct influence of such other components on the gas to be analyzed is generally eliminated in such a manner that the spectral band of radiation to be studied is limited within a wavelength range over which the gas to be analyzed is the only one to absorb. The remaining problem is that other gas components, as pointed out above, have an effect on the absorption of a gas to be measured also as a "distant effect", i.e. by way of collision broadening, whereby the restriction of a measuring band does not help. In collisions, the energy distribution of a molecule is slightly changed, resulting in the broadening of an absorption line. However, the absorbancy value of a molecule integrated across the line remains practically unchanged. The non-dispersive infrared technique is unfortunately incapable of directly measuring absorbancy, but the transmission of infrared radiation is measured over an entire selected wavelength band. Indeed, the transmission of infrared radiation as such is generally a better quantity for representing absorption in practical measuring systems, which employ a wavelength band of a reasonable width, than the absorbancy itself, which is more applicable to cases in which the measuring is done over a very narrow wavelength band and in which the sample chamber is also preferably small.
When measuring non-dispersively over a given wavelength bandwidth, a total transmission Tm will be an integral across a filter spectral range .lambda.1-.lambda.2: ##EQU1## wherein F(.lambda.) is a transmission function for the filter and T(.lambda.) is a transmission depending on the wavelength of a gas sample. Usually the transmission Tm no longer follows the Lambert-Beer law, although this is the case over one wavelength T(.lambda.).
In such a situation that the radiation advances in a long gas-filled conduit, F(.lambda.) represents that wavelength distribution of incoming radiation which falls on a section of the conduit subject to examination. As a result of this, both F(.lambda.) and T(.lambda.) are also functions of place. Hence, the transmission must be monitored also locally in order to understand the signal Tm measured over the entire measuring conduit.
The first aspect of examination is how the wavelength distribution of radiation influences the magnitude of an error caused by collision broadening. As a matter of fact, the magnitude of the error of Tm depends on the bandwidth of a filter, i.e. the very characteristics of F(.lambda.). If measuring is done over a wavelength band of a suitable width, which is in the same order as the width of a single absorption line, the correction requirement of collision broadening is very insignificant or non-existent. This is disclosed for example in EP-publication 0,405,841, wherein carbon dioxide is measured by using a narrow-band source of carbon dioxide radiation. This is equivalent to an optical filter, which includes a plurality of transmission bands corresponding to absorption lines of carbon dioxide. On the other hand, if the optical filter has a transmission band which is narrower than the width of an absorption line to be measured, the collision broadening will cause a reduction of absorption as the peak of the absorption line becomes lower. Such a case is described in WO-publication 94/24528, wherein a single absorption line of oxygen is measured with a highly narrow-band laser diode. Thus, the Lambert-Beer law applies in terms of the laser wavelength and, by scanning the wavelength (which can be done e.g. by changing temperature of the laser diode) over the entire absorption line, it will be possible to calculate the absorbancy across the entire wavelength range and, thus, the collision broadening no longer has an essential effect on the final result.
If the filter has a transmission band which extends over a number of absorption peaks, which is usually the case when measuring carbon dioxide, there is a need for correction since the total absorption increases as the collision broadening increases. This is a common condition in practical measurements.
As already pointed out, the concentration analysis is also affected by the length of a measuring conduit. With a short conduit, the demand of correction is lesser, while with a longer conduit, wherein the absorption of a gas to be measured has reduced transmission substantially more, the demand of correction may be considerable. In the carbon dioxide measurement for the respiratory or alveolar gases of a patient, wherein for example during the course of anesthesia a substantial portion of the gas mixture may consist of nitrous oxide (laughing gas, N.sub.2 O), the calculated CO.sub.2 -gas concentration may be up to 15% too high as a result of the above-described error caused by a transmission integral. Furthermore, if the measuring system involves a measurement of several different gas components in one and the same measuring chamber, the dimensioning of a chamber length is always a compromise, the principal sufferer being the absorbing gas component for which the chamber is hence too long. In this type of situation, the need of correcting the collision broadening is extremely grave, especially if the sum of gas components has been forced mathematically to a certain level, e.g. to 100%, whereby the incorrect concentration analysis of a single gas component affects also the analyses of other gases.